I have a 99 f150 scab lowered with a 2/4 drop. I have this vibration at 60 -65 mph....had the u joints checked and all seemed ok. I was under my truck the other day looking agian and noticed that I can wigle the slip yoke. I suspect this is the cause of the vibration. I talked to a couple of tranny shops...one said I was screwed and get a new tranny and one said this can be fixed if indeed that is causing the vibration. I suspect because the truck was lowered this may have caused this, however they did instal axle shims but maybe not enough? Any body have input on this? Is there anything else that could cause the yoke to wiggle? Like internal stuff? Thanks

 

your drive shaft, it might be bent

 

Truck went to tranny shop today. They said there are no signs of defect. They did say that the rubber seal between the yoke balancer and the yoke was all worn and could possibly be making the vibration. My slip yoke has enough play in it you can feel the wiggle but it doesnt leak. I thought it might be a slip yoke bushing but it doesnt leak..they said to leave it alon....I still think there is to much play in the yoke....I stumped. Yoke or balancer?